20 



HANDBOOK OF PHOTOGRAPHY 



Table III. — Effect on Exposure, Magnification, and Aperture, of 

 Extending Bellows 



Distance between object and 

 lens in multiples of the focal 

 length, / 



Magnification, m, 



Multiply /-number by 



Multiply exposure by 



Distance between lens and filna, 

 in multiples of focal length . . . 



Bellows extension, from infinity 

 position, in terms of focal 

 length 



100/ 



50/ 



20/ 



10/ 



5/ 



3/ 



2/ 



nf 



0.01 

 1.01 

 1.02 

 1.01/ 



0.01/ 



0.02 

 1.02 

 1.04 

 1.02/ 



0.02/ 



0.05 

 1.05 

 1.11 

 1.05/ 



0.05/ 



0.11 

 1.11 

 1.24 

 1.11/ 



0.11/ 



0.25 

 1.25 

 1.56 

 1.25/ 



0.25/ 



0.50 

 1.50 

 2.56 

 1.50/ 



0.50/ 



1.0 

 2.0 

 4.0 

 2.0/ 



1.0/ 



n - \ 



{I + m) = - 



(1 + m)2 = 



(n - 1)2 



nf 



mf = 



= (1 + m)f 



Table IV. — Comparison of Methods of Specifying Apertures 



/-number 



U. S. number. 



5.6 

 2 



16 

 16 



22 

 32 



Measurement of Aperture Ratio. — As the aperture ratio is defined as A = f/D 

 where D is the diameter of the entrance pupil, it is necessary to measure / and D 

 separately. The measurement of focal length is given on page 17. The entrance 

 pupil can be measured by means of an ordinary traveling microscope equipped with 

 an objective having a sufficiently long working distance to reach down into the lens 

 as far as the entrance-pupil plane. Alternatively, the emerging parallel beam from a 



point of light at the focus may be allowed 

 to fall on a piece of photographic printing 

 paper, and the diameter of the disk of 

 light so formed can be measured directly. 

 If the experiment is performed while the 

 lens is mounted upon a nodal slide, both 

 / and D can be determined together. 



Variation of Illumination over the 

 Image. — -The discussion of image illumi- 

 nation given on page 18 refers specifically 

 to the center of the picture. At a field 

 angle 4> from the lens axis, the illumina- 

 tion will be reduced to £'o cos* 0, where Ea is its value on the axis. Two of these 

 cosine terms appear because the oblique-image point is farther from the lens than 



Table V. — Variation of Illumination of Photographic Plate 



Fig. 17. — Oblique illumination in a camera. 



